Optical disks have widely been used as high-capacity information recording media. For the purpose of increasing the capacity of optical disks, the technology thereof has been developed from CDs to DVDs and then to Blu-ray Discs™ by adopting laser beams of shorter wavelengths and objective lenses having higher numerical apertures (NA). Recently, given that such services as cloud computing that utilizes online storage on the Internet have been expanding year by year, a further capacity enlargement of storages including HDDs (hard disk drives) and flash memories is desired.
The following developments are underway with respect to further capacity increase of optical disks.
First of all, in order to reduce a wavelength of a laser beam, a semiconductor laser light source that emits a laser beam in a ultraviolet range of 300 nm to less than 400 nm has been put to practical use. However, because light in an ultraviolet range of 300 nm or less attenuates significantly in the air, reducing the wavelength of the laser beam cannot be expected to provide any great benefits.
As far as increasing NA is concerned, technology has been developed for increasing recording surface density by means of a system that uses an SIL (solid immersion lens) having an NA of one or higher. Also, research has been carried out for increasing recording surface density by means of near-field light that occurs in a region smaller than a diffraction limit of light. In addition, BD-XL among currently commercially-available optical disks has three or four recording surfaces, but current research aims to enlarge the capacity of optical disks by further increasing the number of recording surfaces.
With such ongoing development of increasing the capacity of optical disks, increasing the number of recording surface layers, in particular, could further reduce the amount of signal light to be modulated by reflection by a recording surface of an optical disk, resulting in not being able to secure a sufficient S/N of a reproduction signal. Thus, further increasing an S/N of a detected signal becomes essential in order to keep increasing the capacity of optical disks.
Techniques for further increasing an S/N of a reproduction signal of an optical disk include an optical disk device that uses optical interference. Such an optical disk device using optical interference is disclosed in, for example, Patent Literature 1 or 2. An optical disk device using optical interference obtains large signal amplitude by using reference light to amplify a weak signal amplitude that is obtained by detecting signal light only in a conventional optical disk. First, a laser beam emitted from a laser light source is divided into signal light that is radiated onto an optical disk and reference light that is not radiated onto the optical disk. Subsequently, the signal light reflected off of an optical disk and the reference light reflected off of a reference light mirror interfere with each other. In principle, increasing the intensity of the reference light within an allowable range can further increase an S/N with respect to noise generated in a light detector or an electric circuit.
Furthermore, in the optical disk device, part of the signal light of the laser beam, which is reflected off of the optical disk, generates noise when returning to the laser light source, the laser beam being output from the laser light source of the optical pickup. This noise is generally called “return light noise.” This return light noise is generated when a reflective surface of the optical disk functions as an external resonator with respect to a resonator provided in the laser light source and when the external resonator affects an oscillation wavelength of the laser light source. In a conventional optical disk device that does not use optical interference, return light noise caused therein can be eliminated or reduced by using a combination of optical elements such as a λ/4 waveplate and a polarization beam splitter (PBS), by using a driving current in a laser light source, the driving current being obtained by superimposing a high-frequency current onto a DC current, or by using a self-oscillating laser light source.
However, the conventional optical disk device that does not use optical interference only considers a way to eliminate or reduce return light noise caused due to the relationship between the laser light source and the optical disk, and does not take into consideration return light noise caused by light reflected off of an optical component other than the optical disk. In such an optical disk device that does not use optical interference, return light to the laser light source can easily be inhibited by arranging the optical elements of the optical pickup other than the optical disk at inclinations so as not to be perpendicular to the direction of travel of the laser beam.
The inventors of the present invention had discovered, in the conventional optical disk device using optical interference, that new return light noise is likely to occur due to the relationship between its reference light mirror for reflecting reference light and the laser light source, in addition to the return light noise caused by the relationship between the optical disk and the laser light source. The return light from the reference light mirror could further worsen the S/N ratio.
The conventional optical disk device is described in detail with reference to FIG. 16. FIG. 16 is a diagram showing a configuration of the conventional optical disk device using optical interference. First, return light caused by a laser light source 101 and an optical disk 107 is described with reference to FIG. 16.
A laser beam output from the laser light source 101 is converted into parallel light by a collimating lens 102. The laser beam converted into parallel light passes through a λ/2 waveplate 103. The polarization direction of the laser beam passing through the λ/2 waveplate 103 is rotated by a random angle. The polarization direction of the laser beam passing through the λ/2 waveplate 103 determines the split ratio between the intensities of signal light and reference light that are divided at a PBS 104.
Divided light reflected off of the PBS 104 is the signal light. The signal light passes through a λ/4 waveplate 105 and is then focused on an objective lens 106. The focused signal light is reflected off of a reflective layer 108 of the optical disk 107. The signal light reflected off of the optical disk 107 travels toward the PBS 104 through an optical axis same as that of signal light traveling from the PBS 104 to the optical disk 107. The signal light reflected off of the reflective layer 108 of the optical disk 107 passes through the objective lens 106 again and is converted into parallel light. After passing through the objective lens 106 again and being converted into parallel light, the signal light passes through the λ/4 waveplate 105 again and returns to the PBS 104. At this point, the λ/4 waveplate 105 rotates the polarization direction of the signal light traveling from the optical disk 107 to the PBS 104 by approximately 90 degrees with respect to the polarization direction of the signal light traveling from the PBS 104 to the optical disk 107. The angle of “approximately 90 degrees” is set in consideration of errors in manufacturing the optical elements, errors in placement of the optical elements, and errors in the polarization directions due to the effect of the birefringence in the optical disk or optical elements.
Because the polarization direction of the signal light that has returned to the PBS 104 is rotated by approximately 90 degrees, most of the signal light that has returned to the PBS 104 is transmitted through the PBS 104 and received by an interference light detector 111. However, due to possible errors in the polarization directions as described above, part of the signal light that has returned to the PBS 104 is reflected off of the PBS 104, passes through the optical axis same as that of the laser beam traveling from the laser light source 101, and returns to the laser light source 101 as return light.
The return light generated by the laser light source 101 and a reference light mirror 110 is described next with reference to FIG. 16. The details overlapping with the description of the return light caused by the laser light source 101 and the optical disk 107 are omitted in the following description.
The divided light transmitted through the PBS 104 becomes reference light. The reference light passes through a λ/4 waveplate 109 and is then reflected off of the reference light mirror 110. The reference light reflected off of the reference light mirror 110 travels toward the PBS 104 through the optical axis same as that of reference light traveling from the PBS 104 to the reference light mirror 110. The reference light reflected off of the reference light mirror 110 passes through the λ/4 waveplate 109 again and returns to the PBS 104. At this point, the λ/4 waveplate 109 rotates the polarization direction of the reference light traveling from the reference light mirror 110 toward the PBS 104 by approximately 90 degrees with respect to the polarization direction of the reference light traveling from the PBS 104 toward the reference light mirror 110.
Because the polarization direction of the reference light that has returned to the PBS 104 is rotated by approximately 90 degrees, most of the reference light that has returned to the PBS 104 is reflected off of the PBS 104 and received by the interference light detector 111. However, due to possible errors in the polarization directions as described above, part of the reference light that has returned to the PBS 104 is transmitted through the PBS 104, passes through the optical axis same as that of the laser beam output from the laser light source 101, and returns to the laser light source 101 as return light. Furthermore, because the reflectance of the reference light mirror 110 is extremely higher than that of the reflective layer 108 of the optical disk 107, an extremely large effect of the return light of the reference light is produced.
When the reference light mirror 110 is tilted to shift the optical axis of the reference light as carried out in the conventional optical disk device that does not use optical interference, the optical axis of the signal light reflected off of the optical disk 107 and transmitted through the PBS 104 no longer coincides with the optical axis of the reference light reflected off of the reference light mirror 110 and then the PBS 104. As a result, the effect of amplifying a signal amplitude by optical interference cannot be produced.
On the other hand, the use of a driving current in the laser light source, the driving current being obtained by superimposing a high-frequency current onto a DC current, and the use of a self-oscillating laser light source as in the prior art, expands the line width of an oscillation spectrum of the laser light source, reducing the temporal coherence and consequently the coherence length. Therefore, these methods are not the most appropriate methods for the optical disk device using optical interference.